Gas flow area measurement

ABSTRACT

The effective flow area of a restriction in a gas flow passage, such as that of a gas turbine nozzle or vane, is determined by use of a large reservoir containing pressurized gas which is discharged through the passage in a blowdown operation in which the ratio of back pressure of the restriction to gas pressure in the reservoir is maintained below critical for sonic flow; a timer may be used to measure the time for a predetermined fall of pressure in the reservoir whilst there is sonic flow through the restriction.

BACKGROUND OF THE INVENTION

This invention relates to a method of and an apparatus for determiningthe effective flow area of a restriction in a gas flow passage. Itrelates in particular, though not exclusively, to a method for measuringthe vane (or nozzle) area of a gas turbine engine, and for vane matchingof the vane area of a compressor turbine and a power turbine.

The size of a vane (known also as a nozzle) has a significant impact inthe performance of gas turbine engines because it changes the operatingpoint on the compressor map. Any change in the turbine vane arearematches the engine to a different gas generator speed, massflow andcompressor pressure ratio. For instance, increasing the compressorturbine (CT) vane area while leaving the power turbine (PT) vane areaconstant has the effect of decreasing the gas generator speed, massflowand compressor pressure ratio at constant output power. Conversely,increasing the PT vane area while leaving the CT vane area unchangedcauses the engine to increase in gas generator speed, massflow andcompressor pressure ratio at constant output power.

Vane matching based on effective flow area is a crucial engine overhaulprocedure for predicting optimum engine performance and achievingoptimum efficiency and energy consumption. Improperly matched vanescause poorer-than-expected engine performance, often resulting in enginefailure on test, and increases fuel consumption.

Conventionally, because of the unacceptably high cost and the difficultyof providing a steady sonic rate gas flow, most engine overhaulfacilities u se a flow rig that measures the vane area at sub-sonicflows. In actual engine operation, however, the vanes are (or nearly)choked and the gas velocity at the vane's throat is at (or near) thespeed of sound. Because of its inability to simulate actual sonic flowsthe sub-sonic flow rig provides less accurate and less consistentmeasurement of the vane's area, thus compounding the problem of enginerejects due to incorrect vane match.

SUMMARY OF THE INVENTION

The present invention has as one of its objects the provision of amethod and an apparatus in which the inaccuracy and inconsistencyassociated with conventional techniques for measurement of gas flowthrough a restriction and for vane matching are mitigated or overcome.

According to one aspect of the present invention a method fordetermining the effective flow area of a restriction in a gas flowpassage comprises providing a gas reservoir, arranging said restrictionfor communication with a controlled flow of gas flowing from saidreservoir through flow control, and then operating the flow controlmeans to allow a transient flow of pressurized gas through therestriction from the reservoir, wherein sonic flow is sustainedtemporarily at the restriction by maintaining the ratio of the backpressure of the restriction to the gas pressure in the reservoir belowcritical.

According to another aspect of the present invention apparatus fordetermining the effective flow area of a restriction in a gas flowpassage comprises a reservoir, means for communicating the restrictionwith the reservoir, flow control means operable to control flow ofpressurized gas through the restriction from the reservoir to allowsonic flow to be sustained temporarily at the restriction, pressuresensing means to respond or provide a signal related to the pressure ofgas in the reservoir, and timer means operable to provide a signalrelated to the duration of flow of gas through the restriction.

The method and apparatus may involve timer means operable to measure thetime interval for a predetermined fall of pressure in the reservoirduring flow of gas through the restriction. Alternatively the timermeans may provide a signal which controls is the duration of dischargeflow, and the pressures at the commencement and termination of that flow(or the change of pressure) being measured by the pressure sensingmeans.

The invention accordingly provides within its scope a sonic flow rigwhich may be operated according to the thermodynamic theory applicableto transient blowing down the compressed gas content of a tank reservoirto atmosphere with a vane, nozzle or like restriction located on thereservoir outlet as a restriction to the gas flow. Sonic flow at thethroat of the vane may be generated by keeping the ratio of the backpressure of the vane to the gas pressure in the reservoir below criticalthroughout the duration of the blowdown process. The vane is consideredchoked when the gas velocity at the throat reaches the speed of sound.

In contrast to steady state flow conditions, the sonic flow provided bythe present invention is transient as massflow diminishes with thedecrease in reservoir pressure and temperature during the progression ofthe blowdown process. Nonetheless the flow is sonic and the cost iscomparable to that of a sub-sonic flow rig.

The invention envisages that the transient sonic discharge flow willlast for at least 5 (five) seconds, but will be for less than 80(eighty) seconds. Preferably the sonic discharge flow has a durationbetween 8 (eight) seconds and 15 (fifteen) seconds. In consequence ofthe limited time period for discharge flow, the volume of the reservoirfor flow through a restriction of a cross-sectional area between 5 and25 square inches (between 32.3 and 161 square centimeters) does not needto be greater than 1500 cubic feet (42,475,500 cubic centimeters) for amaximum reservoir pressure of 125 psi (862 kPa).

The method and apparatus of the invention may comprise arranging forflow through a restriction positioned between the reservoir and flowcontrol means, or the restriction may be provided downstream of the flowcontrol means.

The invention teaches use of a reservoir having a maximum operatingpressure of no more than 400 psi (2758 kPa), preferably m more than 125psi (862 Kpa).

The invention is particularly applicable to sonic discharge through arestriction which has a cross-section of at least 1 (one) square inch(6.45 square centimeters) and more typically at least 5 (five) squareinches (32.3 square centimeters). It is envisaged that the area of therestriction may be in a range which extends to 25 square inches (161square centimeters), or even up to 100 (one hundred) square inches (645square centimeters).

It will therefore be appreciated that the invention is applicable tomeasurement of the area of a gas flow restriction which has an areatypical of that found in gas turbine engines.

In at least one of its aspects the invention utilizes the theoreticalconsideration that the mass flow of gas through a converging nozzlereaches a maximum value when the gas velocity at the throat equals thespeed of sound (Mach number is unity or M=I). At this condition, anychange in downstream side of the nozzle cannot propagate to the upstreamside. Further reduction in the downstream back pressure will not affectthe upstream flow rate. The mass flow will remain maximized and thenozzle is said to be choked.

When M=1, the pressure at the throat of the nozzle is called thecritical pressure and the critical-to-stagnation pressure ratio iscalled the critical pressure ratio. For an ideal gas the criticalpressure ratio is a constant, dependent on the specific heats of thegas. Sonic flow through a nozzle is generated when the ratio of the backpressure to the upstream stagnation pressure is equal to or less thanthe critical pressure ratio.

In a tank (reservoir) blowdown system where a nozzle restricts the gasflow, the nozzle is choked when the ratio of the back pressure of thevane to the gas pressure in the tank is less than the critical pressureratio. Assuming an isentropic expansion of the gas, the governingthermodynamic formula for the effective flow area of the vane is:##EQU1## where: A is the effective area of the vane (or nozzle or otherrestriction),

V is the volume of the tank (reservoir),

t is the blowdown time,

pi is the initial absolute pressure of the gas in the tank,

pf is the final absolute pressure of the gas in the tank,

Ti is the initial absolute temperature of the gas in the tank,

R is the specific gas constant,

y is the ratio of the specific heats of the gas.

The vane area may be directly calculated from the above formula bysubstituting the pressure, temperature and time data taken from the flowrun. R and Y are constants specific to the particular gas being used.The volume of the tank should have been previously measured by eitherthe gravimetric or volumetric method.

Alternatively the vane area may be obtained by performing a back-to-backcalibration with a master nozzle that has a previously known effectiveflow area. In this method the master nozzle and the test vane are flowedconsecutively in the sonic flow rig at identical tank initial pressureand final pressure. Since R, y, pi, pf and V remain constant between thetwo runs the effective flow area of test vane can be calculated from thefollowing formula: ##EQU2## where: A is the effective flow area of thetest nozzle,

Am is the effective flow area of the master nozzle,

tm is the blowdown time of the master nozzle,

Tim is the initial temperature of the gas in the tank during the masternozzle run,

tt is the blowdown time of the test nozzle,

Tit is the initial temperature of the gas on the tank during the testnozzle run

The sonic flow rig apparatus of the invention is suitable for measuringthe effective flow area of the vane or like restriction of gas turbineengines at choke conditions. It may consist of a reservoir (tank) thatstores a large volume of compressed gas. The vane may be installed on aholding fixture that is mounted in the discharge pipe very close to thereservoir outlet. The discharge pipe may terminate in a silencer. Thereservoir and discharge pipe may be instrumented with sensitive pressuretransducers and temperature probes. A computer system may be provided toscan the output from the transducers/probes at a very rapid samplingrate.

After charging to pressure the tank may be allowed to blowdown itspressurized gas content by opening a fast acting on-off valve in thedischarge line. While collecting pressure and temperature data thecomputer system may activate a built-in timer when the tank pressurereaches a set initial pressure and deactivates a timer when the tank hasblowndown to a set final pressure. Preferably the preset tank initialand final pressures are kept sufficiently high to ensure that thecritical pressure ratio is not exceeded throughout the blowdown process,thus guaranteeing sonic flow.

If the volume of the tank is previously known the vane area may bederived using the appropriate thermodynamic formula, given the initialtemperature of the gas in the tank, the initial pressure of the gas inthe tank, the final pressure of the gas in the tank and the blowdowntime. The volume of the tank can be measured using gravimetric orvolumetric methods.

If the volume of the tank is unknown the vane area may be obtained byback-to-back flow comparison with a master vane (or nozzle) having apreviously known area. When both vanes are flowed at identical initialand final tank pressures the area and blowdown time of the test vane isdirectly proportional to the area and blowdown time of the master vane.Temperature correction may be applied to offset the small difference inthe initial gas temperature between the test vane run and master vanerun.

BRIEF DESCRIPTION OF THE DRAWINGS

One embodiment of the present invention is now described by way ofexample with reference to the drawings in which:

FIG. 1 is a schematic perspective view of apparatus of the inventionand,

FIG. 2 shows part of the apparatus of FIG. 1 in an exploded state.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A holding tank 1 is a certified ASME pressure vessel that serves as thereservoir for the large volume of compressed gas needed for the flowtest. The tank is equipped in known manner with an inlet valve, outletflange, a manhole, a relief valve, a drain valve and various threadedconnections for the pressure transducers and temperature probes. Theinlet valve is connected by a pipe to a source of clean, dry compressedgas, i. e. a compressed air system or compressed gas tanks.

A main tank shut-off valve 2 is used to isolate the holding tank duringthe installation of the test component or master nozzle. It eliminatesthe need to fully depressurize the tank when replacing the test vane ormaster nozzle, thus preserving the residual compressed gas left in thetank at the end of a test.

Pipe spools 3 act as the discharge pipeline that directs the flow of thelarge volume of gas into a silencer. They are structurally supported andprovided with a retractor that aids in axially moving the discharge pipein order to gain access to the mounting faceplate 4 of the test vane 7.There are also threaded connections for temperature probes T andpressure transducers P. A vent valve is used for relieving the pressurein the test vane section. The ends of the pipe spools are used as themounting flange for the shut-off valve 2, faceplate 4 and on-off valve9.

The faceplate 4 serves as the common mounting surface for the variousvane holding fixtures 5.

The vane holding fixture 5 is responsible for securing the test vane (ormaster nozzle) 7 within the centre of the discharge pipeline. There areseveral holding fixtures in use. Each holding fixture is designed tomatch the dimensional configuration of the vane. The holding fixturesare provided with gaskets to prevent gas leakage through the matingsurfaces of the holding fixture and test component.

Gasketed blanking plates 6 of appropriate configurations are used toseal of any opening in the test component that is not in the flow pathof the gas.

Clamps 8 are used to fasten the test vane 7 to the holding fixture 5.

The fast acting on-off valve 9 controls the blowdown operation of theholding tank 1. It is operated by a pneumatic actuator that facilitatesthe quick opening and quick closing of the valve.

The silencer 10 suppresses the noise generated by the sonic flow. Beingthe point of final discharge to atmosphere, the silencer has to beinstalled in a safe location, preferably outside the building.

The pressure transducers P and temperature probes T measure the pressureand temperature of the gas in the tank 1 and discharge pipe 3. Outputsignals from the transducers are transmitted to the computer system 11via shielded cables.

The computer system 11 controls the main operation of the sonic flowrig. It has hardware and software for signal conditioning, rapid dataacquisition, data reduction, data storage and data print out. It alsosends the output signal that operates the shut-off valve 2 and theon-off valve 9.

In use of the aforedescribed apparatus the sequence of operation startswith the installation of the test vane on the rig. This is accomplishedby installing the blanking plates 6 to the test vane 7, securing thevane 7 to the holding fixture 5 using clamps 8, mounting the holdingfixture 5 to the faceplate 4, retracting the pipe spool 3 to its closeposition and securing the flange connection with nuts and bolts.

The tank is then pressurized by opening the tank inlet valve. Thecomputer system 11 monitors the pressure and temperature in the tank andpipe during the charging process. Once the required tank pressure isreached the tank inlet valve is closed and the tank outlet shut-offvalve 2 is opened.

The blowdown process starts by opening the on-off valve 9 via a startswitch in the computer system 11. The computer system 11 logstemperature and pressure data during the blowdown. It also turns on itsbuilt in timer once the preprogrammed initial tank pressure is reachedand turns off the timer when the preprogrammed final tank pressure isreached, signifying the end of the blowdown process. The computer systemthen closes the on-off valve 9.

From the data acquired the computer system 11 calculates the effectiveflow area of the vane and prints out the test result.

The test vane is removed from the rig by closing the shut-off valve 2,depressurizing the test section by opening the vent valve in thedischarge pipe 3, breaking the flange connection at the test section,opening up the discharge pipe 3 to gain access to the test vane 7 anddemounting the test vane off the faceplate 4.

The above operation is repeated twice when performing a back-to-backcomparison with a master nozzle.

From the foregoing it will be appreciated that the present inventionenables the effective flow area, at the sonic flow condition, of arestriction such as that of the nozzle or vane of a gas turbine engine,to be measured accurately and reliably to the degree hitherto thoughtpossible only by use of expensive equipment which provides for acontinuous sonic flow. The invention further enables comparisons readilyto be undertaken such as for the purpose of vane area matching of thecompressor turbine and power turbine stages of a gas turbine engine.

I claim:
 1. Method for determining the effective flow area of arestriction in a gas flow passage, said method comprising the stepsof:providing a gas reservoir, providing a flow control means, arranginga restriction in communication with a controlled flow of gas flowingfrom said reservoir through the flow control means, and operating theflow control means to allow a transient sonic flow of pressurised gasthrough the restriction from the reservoir, wherein the sonic flow issustained temporarily at the restriction, as a mass flow rate is allowedto diminish rapidly with rapid decrease in reservoir temperature andpressure, by maintaining the ratio of the back pressure of therestriction to the gas pressure in the reservoir below critical. 2.Method according to claim 1, wherein a timer means is provided tomeasure the time taken for the pressure in the reservoir to fall from apreset initial pressure to a preset final pressure.
 3. Method accordingto claim 2, wherein said initial and final pressures are selected toensure that the critical pressure ratio is not exceeded during the flowof gas through the restriction from the reservoir.
 4. Method accordingto claim 1, wherein a timer means is employed to control the periodduring which gas flows through the restriction.
 5. Method according toclaim 4, wherein the pressure in the reservoir is measured atcommencement and termination of said period of gas flow.
 6. Methodaccording to claim 1, wherein the reservoir is of known volume an theeffective flow area of said restriction is derived mathematically usingmeasurements taken related to the change in pressure of gas in thereservoir during a measured period of time and a measurement of thetemperature of the gas.
 7. Method according to claim 1, wherein theeffective flow area of said restriction is derived by comparison of thecharacteristics of flow of gas through said restriction and flow througha comparator restriction of known effective flow area.
 8. Methodaccording to claim 7, wherein a restriction under measurement and acomparator restriction are successively the subject of a flow of gashaving the same initial and final pressures being measured for each flowand utilized to establish the effective flow area of the restriction thesubject of measurement.
 9. Method according to claim 1, wherein thetransient flow has a duration less than 80 (eighty) seconds.
 10. Methodaccording to claim 9, wherein the transient flow has a duration of lessthan 15 (fifteen) seconds.
 11. Method according to claim 1, wherein theinitial pressure in the reservoir before discharge flow is less than 400psi.
 12. Method according to claim 11, wherein said pressure is lessthan 125 psi.
 13. Method according to claim 1 as applied to themeasurement of a gas flow passage restriction which has an effectiveflow area which is typical of that found in gas turbine engines. 14.Method according to claim 1, wherein said restriction has across-section of between 1 (one) and 100 (one hundred) square inches.15. Method according to claim 14, wherein the restriction has across-section greater than 5 (five) square inches (32.3 squarecentimeters).
 16. Method according to claim 1, wherein use is made of aflow control means arranged downstream of the restriction.
 17. Methodaccording to claim 1, wherein use is made of a flow control meansarranged upstream of the restriction.
 18. Method according to claim 1,and as applied to measuring the vane or nozzle area of a gas turbineengine.
 19. Method according to claim 1, and as applied to matching ofthe vane area of a compressor turbine with that of a power turbine. 20.Apparatus for determining the effective flow area of a restriction in agas flow passage comprising:a reservoir, a means for communicating arestriction with the reservoir, a flow control means operable to controla transient sonic flow of pressurized gas through the restriction fromthe reservoir to allow sonic flow to be sustained temporarily at therestriction a pressure sensing means to respond or provide a signalrelated to the pressure of gas in the reservoir, a temperature sensingmeans for providing a signal related to the temperature of the gas inthe reservoir, a timer means operable to provide a signal related to theduration of flow of gas through the restriction, and a processing meansprogrammed to derive a measurement related to the effective flow area ofthe restriction by processing signals from the pressure sensing means,the temperature sensing means, and the timer means.
 21. Apparatusaccording to claim 20, wherein the timer means is operable to measurethe time interval for a predetermined fall of pressure in the reservoirduring flow of gas through the restriction.
 22. Apparatus according toclaim 20, wherein the timer means is operable to control the periodduring which gas flows through the restriction.
 23. Apparatus accordingto claim 22, wherein the pressure sensing means is operable to measurepressure in the reservoir at commencement and at termination of thetimed transient flow of gas through the restriction.
 24. Apparatusaccording to claim 20, wherein the restriction has a cross-section of atleast 1 (one) square inch.
 25. Apparatus according to claim 24, whereinthe restriction has a cross-section of between 5 (five) square inchesand 25 (twenty five) square inches.
 26. Apparatus according to claim 20,wherein the gas flow passage restriction has an effective flow areawhich is typical of that found in gas turbine engines.
 27. Apparatusaccording to claim 20, wherein the reservoir is of known volume. 28.Apparatus according to claim 20, and comprising means for providing asignal related to the temperature of gas in the reservoir.
 29. Apparatusaccording to claim 28, and comprising processing means programmed toderive a measurement related to the effective flow area of saidrestriction by processing pressure, temperature and time information.